Rust preventive film for stretch packaging

ABSTRACT

A rustproof stretch film having an appropriate roll tack, and a self tack adhering to an object to be packaged without peeling when such an object is, for instance, a steel coil. The rustproof stretch film consists of a single layer film or multi-layer film composed of polyolefine resin and includes low molecular weight polyolefin polyol functioning as adhesives, and rust preventive agents, whose overall density is within a range of 0.870-0.935 g/cm 3 .

FIELD OF THE INVENTION

The present invention relates to a polyolefine film for stretchpackaging, and more particularly, to a stretch film having a rustprooffunction favorably used as packaging material for automatic packaging ofa steel coil by stretch packaging apparatus.

The term “stretch film” is used herein to describe a film which has aself-tack property, stretches by application of external force, andshrinks by its elastic recovery force by removing the external force.Such a film is called “cling film (in England)” or “plastic wrap (in theUnited State of America)”. When a stretch film is used as packagingmaterial, its quality characteristic allows packaging of an objecttightly in an orderly form.

Background Arts

Conventionally, packaging materials such as antirust papers, andsynthetic resin films tempered with rust preventive agents have beenused for packaging a steel coil. However, recently, a new technology hasgained attention, in which a rustproof stretch film containingpreservative is utilized so as to make a transparent film withself-adhesive properties (pressure sensitive adherence) andstretchability for automatic packaging of a steel coil.

Japanese unexamined publication No. 2000-326455 discloses a rustproofstretch film comprising three layers in which an intermediate or a corelayer is composed of a low density polyethylene resin with a density of0.920-0.935 g/cm³ mixed with volatile rust preventive agents, a filmcomposed of an ethylene-vinyl acetate copolymer resin is laminated tothe first outer layer (surface layer) of the intermediate layer, and afilm of polyolefine resin is laminated to the second outer layer(surface layer) of the intermediate layer. In this rustproof stretchfilm, one surface of outer layers, which is composed of anethylene-vinyl acetate copolymer resin, functions as an adhesive layer.Moreover, Japanese unexamined publication No. 2001-341238 discloses arustproof stretch film comprising two-layers in which a rustproof layeris formed from materials in which an ethylene-α-olefin copolymer resinwith a density of 0.890-0.920 g/cm³ is mixed with volatile rustpreventive agents, and an adhesive layer is formed from ethylene-vinylacetate copolymer resin containing liquid aliphatic hydrocarbons asadhesive agents.

When a steel coil is automatically packaged utilizing a rustproofstretch film, such a stretch film used as a packaging material has beengenerally taken up to a roll. The width of such a roll (the width of afilm) is normally 25 cm. In an automatic packaging apparatus for a steelcoil, when a roll to which a rustproof stretch film has been taken up isstored in a self-propelled cartridge that rotates along an ellipticorbit passing through a hollow part of a steel coil, the rustproofstretch film in a tape form is continuously pulled out from the take-uproll arranged in the self-propelled cartridge so as to wind around thesurface of the steel coil with its adhesive face inside. The steel coilthat is supposed to be packaged rotates in a fixed direction along anaxis of itself as it interlocks with the rotation of the self-propelledcartridge in the elliptic orbit so that the both end surfaces, externalsurface, and internal surface of the steel coil can be packed with therustproof stretch film in the tape form equally and thoroughly.

The adhesion strength of a rustproof stretch film determines not onlythe operability of the automatic packaging but also whether or notrusting of a steel coil can be successfully prevented. The adhesion of arustproof stretch film is considered to include two different kinds ofadhesion, i.e., a roll tack (adhesion 1) and a self-tack (adhesion 2).

A roll tack involves adhesive resistance at the time when a tape-typefilm is pulled out from a take-up roll arranged in a self-propelledcartridge. The strength of the roll tack represents the strength of thepull out resistance, and therefore, the greater the roll tack is, thegreater the pull out resistance is. Hence, the operability upon theautomatic packaging is closely connected to the roll tack of therustproof film. For example, when the roll tack of the rustproof stretchfilm is excessive, a neck-in which occurs at the time of pulling thetape-type film from the take-up roll associated with the rotation of theself-propelled cartridge becomes large so that the overlapping width offilms becomes small, which may break the film and cause the interruptionof the packaging operation. Further, when the roll tack of the film isdeficient, a change of speed of a self-propelled cartridge during apackaging operation or abrupt deceleration at the end of the packagingoperation causes a film to be pulled out excessively from the take-uproll by the inertia brought by rotation of the take-up roll and tobecome twisted, which prevents continuous operation.

On the other hand, a self-tack of a rustproof stretch film involves theadhesion between films that have packed a steel coil or the like. Thegreater the self-tack is, the firmer the films taken up to a steel coiladhere to each other after packaging. Hence, a self-tack of a rustproofstretch film relates to the rust preventative capacity of the packagingmaterial. Namely, when a rustproof stretch film with a great self-tackis used, the chance that moisture infiltrates into a gap betweenlaminated films becomes less, which allows the preservation ability toperform better. Further, when a rustproof stretch film includes volatilerust preventive agents, not only the volatilization of the rustinhibitor to external environment becomes less but also the infiltrationof the moisture becomes less, so that the preservation ability performseven better.

Conversely, when the self-tack of the film is deficient, the film thatis a packaging material is liable to come off a steel coil, which willlose the satisfactory rust preventative effect of the steel coil.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a rustproof stretchfilm comprising a single or plurality of laminated layers of polyolefineresin having excellent properties in transparency, strength,flexibility, and stretchability, and having roll tack property which isneither excessively great nor small when the film is pulled out from atake-up roll, and an excellent self-tack property which allows the filmto adhere to an object to be packaged without peeling when the single orplurality of laminated layers package a steel coil.

A rustproof stretch film of the present invention comprises a singlelayer structure or a multi-layer structure composed of polyolefineresin, wherein said film comprises the effective amount of low molecularweight polyolefin polyol functioning as an adhesive agent, and rustpreventive agents, and wherein the density of the film is within a rangeof 0.870-0.935 g/cm³ no matter when the film is a single layer or amulti-layer.

A rustproof stretch film of a multi-layer structure according to thepresent invention consists of a two-layer, three-layer, or more thanthree-layer laminated film of resin layers. In this case, one surfacelayer is a low density layer having lower density compared with otherlayer(s). The density of the low density layer is within a range of0.850-0.920 g/cm³, and more preferably, within a range of 0.860-0.910g/cm³.

When a rustproof stretch film of a multi-layer structure has athree-layer structure, it consists of a low density layer, intermediatelayer, and an opposite side surface layer. In this case, the density ofthe intermediate layer is preferably same or greater than that of theopposite side surface layer in order to attain a certain degree of filmstrength and rigidity.

BEST MODE FOR CARRYING OUT THE INVENTION

As a material for a rustproof stretch film of the invention, polyolefineresin is used in which its melt flow rate (MFR) is preferably within arange of 0.3-5.0 g per 10 minutes. The term “polyolefine resin” usedherein includes polyethylene, polypropylene, copolymer of ethylene andα-olefin, or the like. Above all, low-density polyethylene, inparticular, linear low-density polyethylene (LLDPE) is recommended as afilm material in the present invention.

When a rustproof stretch film of the invention has a single layerstructure, the overall density of the film is within a range of0.870-0.935 g/cm³. A density of less than 0.870 g/cm³ makes theformability deteriorate, while a density of greater than 0.935 g/cm³ notonly reduces the bleeding capacity of adhesives and volatile rustinhibitor, but also makes the film too firm and brittle so that thestrength of the film is degraded. Due to similar reasons, the density ispreferably within a range of 0.890-0.925 g/cm³.

Further, when the film is a multi-layer film, the overall density of thefilm is within a similar range of the above.

When a rustproof stretch film has a multi-layer structure, at least onesurface layer consists of a low density layer with a density of0.850-0.920 g/cm³ in order to attain the aforementioned self-tack, andthe density of layers other than this layer is higher than that of thelow density layer. When the film has a three-layer structure, bothsurface layers can consist of low density layers. However, preferably,only one of the surface layers consists of a low density layer.

When the density of the low density layer is less than 0.850 g/cm³, theadhesion becomes too strong, which causes problems in molding such assticking firmly to a take-up roll and guide rolls, and in packagingprocess performed by users. Furthermore, when the density is higher than0.920 g/cm³, it becomes difficult for adhesives or preservatives tobleed out to the surface of film, which will be explained below. Due tosimilar reasons, a preferable range of the low density layer is0.860-0.910 g/cm³.

The layers other than the low density layer have higher density so as tomaintain the overall density of the film in a range of 0.870-0.935g/cm³.

The most preferable combination is the overall density of the film in arange of 0.890-0.925 g/cm³, and the density of the low density layer ina range of 0.860-0.910 g/cm³.

In the multi-layer structure of the rustproof stretch film, lowmolecular weight polyolefin polyol adhesives of the invention must existin the low density layer, which is a surface layer, i.e. a layer havingself-adhesive properties. This layer contacts an object to be packagedsuch as steel coil, and rust preventive agents also exist in this layer.

In this invention, low molecular weight polyolefin polyol is used as anadhesive. As a viscid fluid or wax-type solid at normal temperature,comprising a number average molecular weight of 500-5,000, having anolefinic skeleton, low molecular weight polyolefin polyol is achemically stable adhesive high in saturation, having a hydroxyl groupat both ends of a molecule. It appears on the market under trade namessuch as “POLYTAIL H”, “POLYTAIL HA” (both manufactured by MITSUBISHICHEMICAL CORPORATION) or “EPOL” (manufactured by IDEMITSU PETROCHEMICALCORPORATION).

As low molecular weight polyolefin polyol has higher compatibility withpolyolefine resin, which is a film material, compared to polybutene(also called as polyisobutylene) which has been conventionally used asan adhesive, it does not impair the transparency or strength. Further,as it is easy to bleed on a film surface, a relatively small amount ofagent can provide an appropriate adhesion to the rustproof stretch filmof the invention. That is, the roll tack is neither too less nor tooexcessive, but is in an appropriate range so that the self-tack becomesgreat and desirable.

Although conventional stretch films such as prior arts described abovehave been manufactured by adding adhesives to resin having adhesion uponnecessity so as to use this as film materials, the thus obtained film iseasy to block. That is, while it has an excessive roll tack, self-tackbecomes little after packaging, which demonstrates its poor quality.While low molecular weight polyolefin polyol according to the inventionprovides the film with sufficient self-tack, it also exercises aparticular effect that prevents the blocking mentioned previously. Suchan effect has been confirmed by the present invention for the firsttime.

Moreover, this pressure sensitive adhesive provides flexibility to afilm so as to reduce the brittleness of a film. Therefore, polyolefineresin that has a relatively high density can be used. In addition, dueto its high degree of saturation, the thermostability at the time ofrecovery and reusing of film trimmings is excellent, and degradationproblems at the time of molding can be prevented, which is an advantagederived by the invention.

The amount of low molecular weight polyolefin polyol adhesives to beblended is selected from a range of 0.01-3.0 mass (%) of the whole filmno matter whether the film has a single layer structure or multi-layerstructure.

Although rust preventive agents or rust inhibitors used in the inventionare not limited specifically, volatile corrosion inhibitors thatvolatilize/sublimate at room temperature, or a water soluble corrosioninhibitors are preferable. Especially, volatile corrosion inhibitors aremore preferable. Although there is no restriction about volatilecorrosion inhibitors, nitrite, ammonium compound, urea compound, andmixtures of those are typically used. In particular, the mixture ofnitrite and ammonium compound and/or urea compound is preferable. Asexamples of nitrite, nitrite alkali metal such as sodium nitrite andpotassium nitrite and nitrite alkaline earth metal such as magnesiumnitrite and calcium nitrite are cited. As examples of ammonium compound,ammonium salt such as benzoic acid, phthalic acid, stearic acid,palmitic acid, oleic acid are cited. As examples of urea compound, forinstance, urea, urotropin, and phenyl carbamate are cited.

The compounded amount of the rust preventive agents is selected from arange of 0.05-1.0 mass % regardless of the fact that the film has asingle layer structure or multi-layer structure.

Further, in order to improve packaging aptitude and durability afterpackaging, it is also possible to include an appropriate amount ofslipping agent, anti-blocking agent, electrostatic inhibitor,antioxidant, ultraviolet absorbent, photostable agent, and weather proofagent in any layer within a range not affecting the crystallineproperties of the film to an excessive degree.

Now, a manufacturing method of a rustproof stretch film of the inventionis explained below.

In manufacturing a rustproof stretch film of the invention, asingle-layer or multiple-layer extrusion method can be employed, inwhich molten resin is extruded so as to form a sheet. Typically, asingle-layer or multi-layer extrusion method according to a T-die methodor blown film method is employed. The film can also be drawn in uniaxialdirection or biaxial directions. When a film having multi-layerstructure is manufactured, another method is possible in which a layeris molded by extruding and cooling molten resin, and then another moltenresin is extruded and laminated onto it.

When a rustproof stretch film of the invention has a single layerstructure, low molecular weight polyolefin polyol adhesives and rustpreventive agents are added into resin by using such a method that theseadhesives and preservatives are mixed and melted with resin pellet.

When a rustproof stretch film of the invention has a multi-layerstructure, low molecular weight polyolefin polyol adhesives are mixedwith resin that forms at least one layer while rust-inhibitors are mixedwith the said resin or other resins that forms other layers, so that alaminated film can be formed by the aforementioned various methods. Lowmolecular weight polyolefin polyol adhesives and rust-inhibitors can bemixed with resin that forms either a same layer or other layers.

Further, low molecular weight polyolefin polyol adhesives andrust-inhibitors can be mixed with resins that form each layers,respectively.

There is no restriction about addition layers of adhesives. In amulti-layer structure, adhesives can be added to a low density layer orother layer(s). In the thus obtained film, adhesion appears in a layerwhich has the lowest density (i.e. low density layer) regardless of thelayers in which adhesives are added. Consequently, appropriate adhesionis attained on the surface of the low density layer.

Although the reason why adhesives are prone to bleed out on the surfaceof a low density layer regardless of layers they are added is notdefinite, inventors of the present invention assume that molecularmotion is most active in a low density layer, which brings more gapsamong molecular, so that adhesives are easily taken into the gaps.

Similar to adhesives, rust preventive agents can be added to any resinregardless of the layers it forms.

Among the above multi-layer extrusion methods, it is preferable toemploy a multi-layer blown film process for manufacturing a rustproofstretch film of the invention. Because when rust preventive agents usedin this multi-layer blown film process are volatile, if these areblended with an inner layer film or an intermediate layer film, they arelocated inside of inflated bubbles so as to be surrounded by an outerlayer film. Thus, as there is no risk that the room for manufacturingsuch a film is contaminated by the volatile corrosion inhibitors.

Even if a rustproof stretch film of the invention has single layerstructure or multi-layer structure, any surface treatment, such ascorona discharge treatment, can be applied on the film, if necessary.

Although the thickness of a rustproof stretch film of the invention canbe determined arbitrarily, generally, it is of thickness of 40-100 μm,which is used commonly for a packaging material for a steel coil.

When a rustproof stretch film of the invention has a three-layerstructure, the thickness ratio of these layers are determined within arange of a=1-7, b=0.5-3, wherein the low density layer/the intermediatelayer/the opposite side surface layer =1/a/b, by comprehensivelyconsidering the formability, gained effect of adhesion, strength,flexibility, and tightness of the steel coil packaging. When such a filmhas a two-layer structure, the thickness ratio of the both layers isdetermined within a range of 6/1-1/6.

Now, examples and comparison examples are shown to explain a rustproofstretch film of the invention in detail. It will be appreciated thatthese examples do not limit or restrict the technical scope of theinvention.

Namely, characteristics of each stretch film obtained by each exampleand comparison example described below are evaluated by the followingmethod:

(Roll Tack: Adhesion 1)

Each stretch film with a single- or multi-layer structure has been takenup in a roll in such a manner that a surface having less adhesion isinside. The end of the wound film has been pulled by a spring balance tomeasure its tension as a roll tack.

(Self-Tack: Adhesion 2)

In a thermo-hygrostat room with 23° C. and the relative humidity of 50%,a stretch film has been cut into two pieces 200 mm in length X 50 mm inwidth in a longitudinal direction of the film. One film with greatersurface adhesion has been overlapped with the other film with lesssurface adhesion, with an overlapping area 50 mm X 50 mm, to place aweight of 2 kg thereon and leave for an hour with the same temperatureand humidity condition. Then, the film has had shear-tension applied ina longitudinal direction by a TENSILON, one of the Tension/CompressionTesting Machines (Toyo Seiki Seisaku-sho, Ltd.) with an initial clamplength of 100 mm. The maximum tensile strength is determined as a selftack and the elongation percentage is calculated by dividing theelongation at the time of maximum tensile strength by the originallength; then, the value is multiplied by 100. Further, the product inwhich a self-tack is multiplied by the elongation percentage is definedas a self-tack index so as to represent a self-tack level.

(Rust Preventive Effect)

A length of cold-rolled steel (JIS G 3141 SPCCSD 60 mm X 80 mm X 2.3 mm)has been placed in a bag made from a stretch film and sealed by heatsealing. The thus obtained sealed sample has been stored in an ovenwhose inside atmosphere has been changed as described below to evaluatethe rusting situation of the above cold-rolled steel after 24 hours:

-   -   10° C. 80% RH 3 hr    -   50° C. 95% RH 3 hr    -   10° C. 80% RH 3 hr    -   50° C. 95% RH 15 hr

The following is the performance index: ⊚: no rusting, ◯: a few spotrusting (1-3 spots), Δ: several spot rusting (4-10 spots), ▴: many spotrusting (more than 10 spots), X: overall discoloration/face rusting

(Prick Test)

The test has been performed pursuant to JIS Z1707:

-   -   The end R of the pricking jig: R=0.6 mm    -   The pricking speed: 50 mm/min.    -   The film size: 45 mmΦ

(Measurement of Film Surface Adhesives) Analytical method: X-rayPhotoelectron Spectroscopy (XPS) Instrument model: ESCA750 manufacturedby SHIMADZU CORPORATION X-ray Source: Mg Kα (8 kV 30 mA) Photoelectronuptake angle: 90° Analytical diameter: 10 mm Φ Spectral RegionDefinitions: {circle over (1)} Cls (280-295 eV) {circle over (2)} Ols(525-540 eV)

As to a rustproof stretch of the present invention, it is preferablethat the O/C value measured pursuant to XPS method is within a range of0.1-5.0 atm %.

EXAMPLE 1

A multi-layer blown film forming equipment has been used to make arustproof stretch film of three-layer structure of 60 μm thickness underthe condition of the extrusion resin temperature of 180, the blow-upratio of 2, and the thickness ratio of each layer 1/1/1 by using threekinds of film materials (resins) as described below:

Internal Layer: Volatile rust preventive agents have been blended withan equalizing mixture of an LLDPE (density: 0.920 g/cm³, MFR: 1.0 g/10min.) and another LLDPE (density: 0.902 g/cm³, MFR: 11.0 g/10 min.) inthe proportion of 0.3 mass % to the whole multi-layer film, while thecompounded amount of low molecular weight polyolefin polyol adhesives(manufactured by MITSUBISHI CHEMICAL CORPORATION, the trade name:POLYTAIL HA) has been changed as shown in table 1.

Intermediate Layer: Except for the omission of the blending of rustpreventive agents and adhesives, the same LLDPE for the internal layerresin has been used.

External Layer: an LLDPE (density: 0.875 g/cm³, MFR: 3.0 g/10 min.) hasbeen used.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that the surface of the internal layer showedhardly any adhesion but the surface of the external layer showedadhesion.

The film surface having adhesive mixing ratio of 0.5% has been analyzedby using XPS and obtained the O/C value of the surface of the externallayer of 2.7 atm %, and the O/C value of the surface of the internallayer of 0.4 atm %. This result demonstrates that a certain amount oflow molecular weight polyolefin polyol exists in the external layersurface, and less in the internal layer surface.

(COMPARISON EXAMPLE 1)

Except for the amount of adhesive agents blended with the internal layerresin used in example 1 being changed to the proportion of 0.5 mass % tothe whole multi-layer film, and no rust preventive agent being blended,the same internal layer resin, intermediate layer resin, external layerresin used in example 1 have been used under the same conditions asexample 1 to obtain a stretch film of three-layer structure.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that the surfaces of the internal layersshowed hardly any adhesion but the surfaces of the external layersshowed adhesion. We also observed that a film according to this exampleindicated greater roll-tack and self-tack compared to example 1.

(COMPARISON EXAMPLE 2)

Each resin component of the internal layer resin, intermediate layerresin, and external layer resin used in example 1 have been replacedwith an LLDPE (density: 0.926 g/cm³, MFR: 11.0 g/10 min.), while theamount of adhesives blended with the internal layer resin has beenchanged to the proportion of 0.5 mass % to the whole multi-layer film,and the amount of rust preventive agents has been changed to theproportion of 0.3 mass % to the whole multi-layer film. Except for thosechanges, the same condition as example 1 has been maintained to obtain arustproof stretch film of the three-layer structure.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that both the roll-tack and self-tack becameextremely small.

(COMPARISON EXAMPLE 3)

The adhesive agents blended with the internal layer resin in example 1have been replaced with polybutene, and the amount of composition hasbeen changed as shown in table 1. Except for those changes, the samecondition as example 1 have been maintained to obtain a rustproofstretch film of the three-layer structure.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that the adhesion between the surfaces of theinternal layers and between those of the external layers showed thesimilar tendency to example 1. However, the roll-tack became greater andthe self-tack became less when compared with example 1.

The characteristics of each multi-layer stretch film obtained by example1 and comparison examples 1-3 have been evaluated in accordance with thepreviously described method. The results are shown in table 1:

In the density of table 1, “total” represents the density of the wholefilm, and “external layer” represents the density of the external layerwhich corresponds to the low density layer of the multi-layer structurefilm.

EXAMPLE 2

A multi-layer T-die film forming equipment has been used to make arustproof stretch film of three-layer structure of 60 μm thickness underthe condition of the extrusion resin temperature of 240%, and thethickness ratio of each layer is 1/1/1 by using three kinds of filmmaterials (resins) as described below:

The term “contact layer resin” is used herein to indicate a resinextruded from a T-die to form a film layer which directly contacts acooling roller, and the term “non-contact layer resin” is used toindicate a resin extruded from a T-die to form a film layer placed onthe opposite side of the above film layer having an intermediate layerfilm therebetween.

Contact Layer Resin: An equalizing mixture of an LLDPE (density: 0.912g/cm³, MFR: 3.8 g/10 min.) and another LLDPE (density: 0.902 g/cm³, MFR:3.0 g/10 min.) has been used.

Intermediate Layer: The identical resins used in the above contact layerhave been used. The identical rust preventive agent used in example 1 inthe proportion of 0.3 mass % to the whole multi-layer film has beenblended, and the identical low molecular weight polyolefin polyoladhesives have been blended with the layer. However, the compound ratioof the adhesive agents to the whole multi-layer film has been changed asshown in table 2.

Non-Contact Layer Resin: The identical LLDPE used for the external layerresin in example 1 has been used.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that the surface of the contact layer sideshowed hardly any adhesion but the surface of the non-contact layer sideshowed adhesion. Further, the roll-tackiness became less, but theself-tackiness became greater when compared with the multi-layer film ofexample 1.

EXAMPLE 3

A multi-layer blown film forming equipment has been used to make arustproof stretch film by replacing the LLDPE resin (density: 0.920g/cm³, MFR: 11.0 g/10 min.) for the internal layer and intermediatelayer used in example 1 with another LLDPE resin (density: 0.926 g/cm³,MFR: 11.0 g/10 min.), and also replacing the LLDPE resin (density: 0.875g/cm³, MFR: 3.0 g/10 min.) for the external layer with another LLDPEresin (density: 0.908 g/cm³, MFR: 11.0 g/10 min.). Further, rustpreventive agents have been blended with an intermediate layer resin,while adhesives have been blended with the internal layer resin and/orthe intermediate layer resin. The compound ratio to the wholemulti-layer film has been changed as shown in table 2. Except for thosechanges, the identical conditions of example 1 have been maintained tomake a rustproof stretch film of a three-layer structure.

As the results of the examination of the adhesion of the thus obtainedmulti-layer film, we found that both the roll-tack and self-tack becameslightly less compared with example 1, wherein the more adhesives havebeen blended with the intermediate layer resin, the lower the roll-tackhas become, and the greater the self-tack has become.

The characteristics of each multi-layer stretch film have been evaluatedin a similar manner of example 1, and its results are shown in table 2:

(EXAMPLE 4)

A multi-layer blown film forming equipment has been used to make arustproof stretch film in which the two kinds of the LLDPE resins forthe internal layer used in example 1 has been used for one layer(A-layer) with mass ratio of high density resin/low density resin=80/20, while the LLDPE resin for the external layer used in example 1has been used for the other layer (B-layer). Further, rust preventiveagents and adhesives have been blended with the A-layer resin. Thecompound ratio to the whole multi-layer film has been changed as shownin table 2. The thickness of the A-layer and B-layer has been changed to30 μm, respectively. Except for those changes, the identical conditionsof example 1 have been maintained to make a rustproof stretch film of atwo-layer structure.

As the results of the examination of the adhesion of the thus obtainedfilm, we found that the roll-tack became slightly greater while theself-tack became slightly less compared with example 1. However, thethus obtained film was sufficient to use.

The characteristics of each multi-layer stretch film have been evaluatedin a similar manner of example 1, and its results are shown in table 2:TABLE 1 Density Adhesives Rust (g/cm³) Com- pre- Roll Prick Rust Ex-pound ventive Tack Self-Tack Energy Preventive ternal ratio agents (kg/Adhesion (a) Elongation Index Strength Elonga- (Strength × Effect ItemTotal Layer Kind (%) (%) 250 mm) (N/50 mm) (%)(b) (a × b) (N) tion (%)elongation) 10 days Example 1 0.904 0.875 Polyol 0.50 0.3 2.5-3.5 25.0380 9,500 3.0 65 195 ⊚-◯ 0.30 0.3 4.0-5.0 22.0 300 6,600 3.0 65 195 ⊚-◯0.15 0.3 4.5-5.5 19.0 35 665 3.0 65 195 ⊚-◯ 0.10 0.3 4.5-5.5 18.0 25 4503.0 65 195 ⊚-◯ None 0.3 5.0-6.0 17.0 10 170 3.0 60 180 ⊚-◯ Comparison0.904 0.875 Polyol 0.50 0 5.5-6.5 28.0 450 12,600 3.0 65 195 X Example 1Comparison 0.927 0.926 Polyol 0.50 0.3 0.7-0.9 3.5 4 14 3.3 60 198 ◯Example 2 Comparison 0.901 0.875 Polyol 0.90 0.3 4.0-5.0 16.0 16 256 2.760 162 ⊚-◯ Example 3 0.50 0.3 4.5-5.5 14.0 15 210 2.7 60 162 ⊚-◯ 0.300.3 5.0-6.0 17.0 14 238 2.7 60 162 ⊚-◯ 0.20 0.3 5.3-6.3 17.0 10 170 2.760 162 ⊚-◯

TABLE 2 Density (g/cm³) External Rust Self Tack Layer Adhesives pre-Elon- Prick Rust (Non- Com- ventive Adhesion gation Elon- EnergyPreventive contact pound agents Roll Tack (a) (N/ (%) Index Strengthgation (Strength × Effect 10 Item Total Layer) Kind ratio (%) (%)(kg/250 mm) 50 mm) (b) (a × b) (N) (%) elongation) days Example 2 0.9010.875 Polyol 0.30 0.3 2.4-3.4 26.0 400 10,400 3.8 55 209 ⊚ 0.10 0.32.7-3.8 20.0 130 2,600 3.7 55 204 ⊚ 0.05 0.3 3.2-4.3 19.0 60 1,140 3.755 204 ⊚ None 0.3 4.8-5.8 18.0 15 270 35 40 140 ⊚ Example 3 0.921 0.908Polyol 0.5 0 0.3 2.1-2.5 18.0 220 3,960 3.1 60 186 ⊚-◯ 0.2 0.3 0.32.3-2.7 20.0 270 5,400 3.1 60 186 ⊚-◯ 0 0.5 0.3 2.7-3.0 22.0 320 7,0403.1 60 186 ⊚-◯ Example 4 0.895 0.875 Polyol 0.50 0.3 3.0-4.0 22.0 3507,700 2.9 65 189 ⊚-◯ 0.30 0.3 4.3-5.3 20.0 270 5,400 2.9 65 189 ⊚-◯ 0.150.3 4.7-5.5 18.0 32 576 2.9 65 189 ⊚-◯

INDUSTRIAL APPLICABILITY

A stretch film according to the invention can be freely applied to anypackaging machinery for high-speed packaging or normal-speed packaging.Further, rust preventive agents or adhesive agents can effectively bleedto a surface layer of the film, which results in a great reductioneffect. Moreover, as mechanical strength can be maintained, a costreduction effect is also expected. Furthermore, in a rustproof stretchfilm of a multi-layer structure, when a volatile corrosion inhibitor isused as a rust preventive agent, blending of the inhibitor is performedin an internal or intermediate layer with multi-layer blown filmmolding, and an intermediate layer with multi-layer T-die molding, whichallows the volatile corrosion inhibitor to vaporize during the filmmaking process so that the contaminant in the manufacturing room can bereduced.

1. A rustproof film for stretch packaging comprising a single layerstructure or a multi-layer structure composed of polyolefine resin,wherein said film comprises effective amount of low molecular weightpolyolefine adhesives and rust preventive agents, and wherein thedensity of the film is within a range of 0.870-0.935 g/cm³.
 2. Arustproof film for stretch packaging defined in claim 1 comprising atwo-layer structure, wherein one of surface layers is a low densitylayer whose density is within a range of 0.850-0.920 g/cm³, and theother of surface layers has greater density wherein the adhesives andrust preventive agents exist in the low density layer.
 3. A rustprooffilm for stretch packaging defined in claim 2 wherein a density of thelow density layer is within a range of 0.860-0.910 g/cm³.
 4. A rustprooffilm for stretch packaging defined in claim 2 wherein a density of thewhole film is within a range of 0.890-0.925 g/cm³.
 5. A rustproof filmfor stretch packaging defined in claim 2 wherein a density of the lowdensity layer is within a range of 0.860-0.910 g/cm³ and a density ofthe whole film is within a range of 0.890-0.925 g/cm³.
 6. A rustprooffilm for stretch packaging defined in claim 1 comprising a three- ormore layer structure, wherein one of surface layers is a low densitylayer whose density is within a range of 0.850-0.920 g/cm³, and theother of surface layers and one or more intermediate layers have greaterdensity wherein the adhesives and rust preventive agents exist in thelow density layer.
 7. A rustproof film for stretch packaging defined inclaim 6 wherein a density of the low density layer is within a range of0.860-0.910 g/cm³.
 8. A rustproof film for stretch packaging defined inclaim 6 wherein a density of the whole film is within a range of0.890-0.925 g/cm³.
 9. A rustproof film for stretch packaging defined inclaim 6 wherein a density of the low density layer is within a range of0.860-0.910 g/cm³ and a density of the whole film is within a range of0.890-0.925 g/cm³.